Merge remote-tracking branch 'pr/237' into develop

Twitter Feature on Main-View from @loadtocode

Twitter feature within home/index of LEAP web app as feed of tweets of one user's twitter account

How to use it: set up an Application-only authentication by twitter (https://dev.twitter.com/oauth/application-only) and then create a config/secrets.yml including:
```
development:
  twitter:
    enabled: false # set to true for usage
    twitter_handle: XXXXX #put your twitter handle here
    bearer_token: XXXXX #put your bearer token here
test:
  twitter:
    enabled: false # set to true for usage
    twitter_handle: XXXXX #put your twitter handle here
    bearer_token: XXXXX #put your bearer token here
```

things in the PR:

- [x] gemfile:twitter
- [x] twitter helper (caching (to avoid reaching twitter rate limits)
- [x] index twitter view
- [x] twitter css (twitter design as discussed in call: Profil-Header: Picture, Link to twitter + twitter_handle; removed picture and twitter_handle from individual tweet, heading aligns at the top, only the last 3 tweets are shown)
- [x] script to generate bearer token
- [x] script to invalidate generated bearer token
- [x] documentation (Readme + Doc)

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