[3.0] Correctly handles quotes in mentions, plus other entity-related fixes

[Sesquipedalian]

Fixes #8820
Closes #8837

• The underlying issue that made this more difficult to fix in 3.0 was that member names were being saved with unescaped apostrophe characters rather than ' entities, but the mention code assumed the entities would be used. This issue affected not only member names, but various other things that are processed via Utils::htmlspecialcharsRecursive(). In previous versions of SMF, that function always forced the ENT_QUOTES flag to be used, but in 3.0 it was changed to allow the caller to set the flags and to use ENT_COMPAT by default, just like Utils::htmlspecialchars() does. This issue has been fixed by specifying ENT_QUOTES in all calls to Utils::htmlspecialcharsRecursive() where it was expected.

  • NOTE TO DEVS, TESTERS, AND EARLY ADOPTERS: This PR does not magically replace any unencoded apostrophe characters in your database with ' entities. You will need to fix them yourself.

• Makes several other improvements to our handling of HTML entities, including:

  • Adds $double_encode param to Utils::htmlspecialchars() and friends
  • Adds SMF\Utils::htmlspecialcharsDecodeRecursive()
  • Simplifies entity processing methods. This one adds some polyfills for the various grapheme_* functions from the intl extension, and then uses them to simplify code in the SMF\Utils::entity* methods.

• This PR also updates jquery.atwho.min.js to version 1.5.4. Doing this made it unnecessary to add the custom override that was added to mentions.js in [2.1] Fix quotes in names for mention handling #8838. We may want to update jquery.atwho.min.js in SMF 2.1 as well so that we can get rid of that customization there, too.

• Finally, based on all the changes above, Correctly handles quotes in mentions

[jdarwood007]

It's fine to update at.js for now. @live627 commented in #8132 that the theme PR #7933 will replace this with something native.