This introduces new Follow menu items for linking to our BlueSky,

LinkedIn, and Mastodon social media accounts, giving any users who

happen to click that menu a better idea of where we're active.
It also changes the icons to use Tabler-based icons instead of Font

Awesome, which ensures we have consistent icon sizes (font-based icons

are limited to the glyph size).

Tested the menu and all the links in it.