Rename `betty.ico` to `favicon.ico`

[bartfeenstra]

Rename betty.ico to favicon.ico, or more specifically: ensure sites are generated with a favicon.ico instead of/in addition to a betty.ico. This is because some clients (such as Firefox) request /favicon.ico for every page by default, causing loads of 404 Not Found errors.

Definition of Done

• Rename betty.ico to favicon.ico
• Remove all favicon.ico workarounds

Blocked by

• The 0.4.x branch being opened