User Mentions Improvements by Xon 2.10.3

• Fix minor php 7.3+ compatibility issue (optional admin triggered task to rebuild permissions)

• Extend user's Account Privacy saving process to also save email options

• XF2.1.1 compatibility bugfix

• Prevent mixing protected/public when extending entities

• XF2.1 support

• Add CLI command to rebuild permissions (php cmd.php xf-rebuild:sv-permissions)
• GUI option to kick off a permission rebuild
• Fix that unsetting a individual permission entry didn't trigger a permission rebuild
• Rebuild change-tracking when rebuilding permissions to be more accurate and only scoped to user-group changes

• Prevent erroring when User Essentials is rebuilding user mentions and a post without a thread is encountered.

• Fix a memory leak when running from a long running process (effects User Essential's mention tracking rebuild)

• Fix error "src/addons/SV/Utils/InstallerHelper.php Unexpected contents" by updated shared installer code to the non-buggy build

• Fix typo in email footers