#19 Change "Fellowship" to "supporter": texts and buttons

Fixes #6 largely: - [x] Add "Become a supporter" to the texts-**.xml files - [x] Add the latest fontawesome fonts to enable some additional icons - [x] Remove "Join" from the menu bar, and add "Become a supporter" to the bar next to "Login" or "Change language" - [x] Index: Change green button text to from "Join the FSFE" to "Become a supporter" - [x] "followup" box below every page: Headline to "Become a supporter" - [x] "followup" box: change text to text "By becoming a supporter of the FSFE you empower our work and help to assure our financial independence." (need approvement of pr@) - [x] Add translations - [x] remove /index.*.xhtml pages which are not up-to-date by then This is still work in progress and requires some coordination with translators@. Also, we could combine this with a little clean-up of the static texts files.