Full-text content syncing

Full-text content is now synced to the server and other clients. Up to
~500K of content is transferred with each sync, so multiple syncs may be
necessary. Downloaded content is written to temporary files and
processed on idle (currently 5 seconds), so synced content may not be
immediately available.

Performance will improve with API syncing and async DB in 4.1.

This also includes some other full-text tweaks, such as storing
text converted from HTML in cache files, which may speed up phrase
searching.
Don't truncate HTML file content in full-text sync

Also write a cache file for HTML files at index time
Fix index rebuild
Better handling of maxLength and HTML

- Don't truncate before HTML conversion
- Correctly calculate indexed chars and total chars
- Move HTML conversion code into one function
Order by clientDateModified DESC when getting content to upload

This way content from newly added/modified items will get uploaded
before content from older items.
Add pref to disable full-text syncing, and prompt existing users on sync

FT sync is enabled by default for new users and configurable in the Sync
prefs.

Also disable downgrades once full-text sync is enabled, since otherwise
someone switching back and forth between old and new versions could miss
full-text content updates.
"other computers" => "other linked devices"
Back up database before disabling downgrade
Localize full-text sync strings
Merge branch '4.0_fulltext_sync' into 4.0
Update versions, repotime, and submodules
Update locales from Transifex
Don't reindex downloaded full-text content if already up to date
Bind fulltext shutdown listener to this
Fix error dragging items from My Library to group library
Update versions
Don't send full-text content in read-only libraries
Update to citeproc-js 1.0.502
Apply Fx25 tree-row fix to Windows only, and fix spacing there too

"height: auto" (from another extension's fix for this) actually caused
the bottoms of tree rows to be cut off on Windows (and at least on one
Linux installation with Chinese characters).
Do not show SSL error message for other network errors