Add rel="me" for identity verification on external sites

[dsevillamartin]

Fixes #60 Fixes #61

Changes proposed in this pull request:

• Add option for rel="me"
• Update link and add help text for FontAwesome v5 icons

Screenshot

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).