Support Multisite

[mlwilkerson]

Closes #127

TODO:

• [x] in maybe_move_release_metadata_for_upgrade() clean up obsolete font-awesome-releases transient or site-transient (it's a normal non-transient option now).
• [x] in maybe_move_release_metadata_for_upgrade() clean up obsolete font-awesome-last-used-release site-transient. It's now a transient, not a site-transient (confusing language, but it's a site-specific bit of data, no longer network-wide)
• [x] handle plugin initialization for sites created after initial network activation
• [x] add a "finally" clause and more to the for_each_blog()
• [x] update for_each_blog() according to this guideline from docs on restore_current_blog()

  restore_current_blog() should be called after every switch_to_blog()

• [x] throw exception on activation for multisite when WP version is < 5.1.0 (because that's when the wp_initialize_site hook was introduced
• [x] maybe use wp_get_sites() in for_each_blog(), but consider the issue of what happens when there are many sites.
• [x] fix multisite tests in CI
• [x] determine whether mysql 5.7 is incompatible with multisite because of delete_expired_transients() failing in WP Core (works with mariadb 10.5.8)
• [x] test an upgrade on a non-multisite installation
• [x] test path/subdirectory-based multisite

[mlwilkerson]

A pre-release of the plugin within support for multisite is available, from this branch: https://github.com/FortAwesome/wordpress-fontawesome/releases/tag/4.3.0-1

The current deployment plan is to release 4.2.0 to the plugins directory next week. 4.2.0 will not include this branch, so no multisite support.

After getting some feedback on the 4.3.0-1 pre-release, if it seems ready, it would be released to the plugins directory in the following weeks.