Update / rethink source list view

[alexhebing]

Now that @GuillaumeLNB is able to upload sources automagically (through a script), the number of sources will increase quickly. He has announced that in a few months thousands of sources will be uploaded. This calls for a rethinking of the source list view, which now displays a list of all sources rather naively.

Firstly, it might be nice to implement this as part of #179 (and thinking along the lines of #163). There should in any case be some sort of 'Annotate' link and page, where users see the last 5 (or so) sources they annotated, and search through all available sources somehow.

Secondly, implementing the mentioned searching could potentially be combined with #208 and #176, i.e. storing and loading the source text (and perhaps titles) in an ES index.

[jgonggrijp]

Note to self and @GuillaumeLNB: discuss testimonial splitting next week.

[jgonggrijp]

Somewhat related: #277.

[jgonggrijp]

Closing in favor of #322 and #163.