Pale Moon 27.1.0

# Changes/Fixes:
- Reworked the media back-end completely (thanks Travis!) to use FFmpeg (including support for FFmpeg v3 and MP3 playback) and our own MP4 parser, and no longer relying on gstreamer on Linux, as well as adding some improvements on Windows for media parsing and playing.
- On Linux, Apple .mov files of the correct type will also be played through FFmpeg now, for those rare occasions where they are still in use, considering there is no Quicktime plug-in available on that operating system.
- Restored the classic about:config styling.
- Added a fallback to US-ASCII if the autoconfig UTF-8 conversion fails.
- Improved cross-compartment wrapper handling when managing a large number of tabs (fixes a performance regression with v27).
- Changed the way audio and video synchronization is calculated to account for (slow) device latency, preventing things from getting out of sync on e.g. BlueTooth-connected speakers.
- Changed the way scripts are handled when they are stopped from the "unresponsive script" dialog, to prevent browser lockup. We will now stop all scripts in the affected compartment in one go.
- Fixed several errors in the devtools.
- Fixed a nasty crash caused by cross-origin referrers.
- Fixed the installer to allow 64-bit versions of the browser to be installed on Vista again.
- Added HTML5-spec clipboard handling for content (cut&copy only -- paste is not allowed for security reasons).
- Made multiple changes to the toolkit jetpack modules to cater to PMkit extensions.
- This should make running SDK-based modules as PMkit extensions fairly simple for extension developers. See the introductory text to these release notes.
- Fixed a css layout issue: make max-width affect contributions to intrinsic min-width.
- Implemented several updates to the permissions manager. Among others, Improved the permissions manager (about:permissions) with a more complete set of permissions for pages.
- Removed otherwise unused Metro browser platform/widget code.
- Removed support for non-standard/deprecated let blocks and expressions.
- Made the use of let as a keyword versionless and ES6 compliant.
- Made the privacy category in preferences a tabbed setup to better fit the current options.
- Fixed a regression preventing certain MP4 video files from playing.
- Fixed a regression where seeking in media files would halt playback/jump to the end of the stream.
- Fixed a crash caused by certain downloadable fonts with DirectWrite in use.
- Improved downloads-button indicator legibility on some combinations of Windows versions and system theme colors.
- Changed the Facebook user-agent override to be our native one, based on reports from users that it is (finally) working acceptably.
- Fixed site-specific useragents being ignored if a global override is defined.
# Security/privacy changes:
- Changed CORS handling to allow data: sources, assuming they are same-origin. This should fix the infamous "Facebook endless reload" issue and may make some other sites that assume this particular (unspecified) CORS behavior happy with Pale Moon.
- Reinstated the network.stricttransportsecurity.enabled preference so people who choose privacy over HSTS can do so again.
- Added, In HSTS "off" state, prevention of HSTS site status from being written to disk.
- Updated the IDN blacklist with more extended unicode characters that "look very similar to" normal ASCII characters, to prevent spoofing of well-known domains. If blacklisted characters are found, the IDN domain name will be displayed in its punycode form.
- Fixed an exploitable crash when using MP4 video.
- Fixed an exploitable crash in XSL parsing.
- Fixed a potential security issue when exporting certificates with specially-crafted credentials.
- Fixed a potential use-after-free situation in frame selection.
- Fixed a leak of window details through the Ion compiler in certain situations.
- Fixed the potential for an exploitable crash involving Javascript GC. DiD
- Fixed a potential overflow situation in (non-released) WebRTC code. DiD
- Fixed a potentially unsafe situation in websockets. DiD
- Fixed several memory and other safety hazards.