Clean up file/folder organisation

[Treora]

The organisation of this repo has become a bit messy over time. I intend to move some things around soon, currently thinking about these changes:

• [ ] Make clearer which code is an 'extension page' (overview, options, page-viewer), what is background or content script, and what is commonly usable by any of them. Be more consistent in using (or possibly remove) the separation of background and content_script folders, it is a bit confusing now.
• [x] (124319d) Move html files out of extension, into src. Rename bundled scripts (e.g. src/overview/main.jsx) so they match the names used in html script tags (also hard-wire output naming in gulp task). extension is to be more like a 'build' output folder. (where do we put images?)
• [x] In src/overview, separate 'application' (main.jsx + store.js) and 'redux module' (the rest). Maybe make 'application' part reusable for e.g. options page.
• [x] Move stuff from src/search/find-pages into src/page-storage, it's not really search-related.
• [x] Consider altogether regrouping activity-logger, page-analysis, page-storage, and search, or at least define their roles and interfaces better.
• [x] Refactor generateVisitDocId functions etc in some sensible way.
• [x] (c73fb7e) Remove unused code: src/browser-history, ...more?
• [x] (354b7dc) Move src/freeze-dry into its own repo (or else into util).

[poltak]

(where do we put images?)

Could put them in sub dirs of the main module's dir, like src/overview/img, reflecting the build extension/overview/img and just add entries for them into staticFiles in the gulpfile. Or have another top level dir for static assets mirroring the src/ and extension/ structures, like assets/{overview,options}/{fonts,img}/.

HTML files could be treated as static files in the same way, or could unify them in a shared template and spit them out for their respective build tasks using something like gulp-template.

[Treora]

Thanks still for the input. For now the question about images is solved, since there are no images anymore (using semantic-ui's icons and ditched the background). The question will probably pop up again though.

I started cleaning up in the 'cleanup' branch, checking at least some of the boxes.

[Treora]

Move html files out of extension, into src

This one has been done in 124319d. The other tasks may have to wait a bit; upcoming feature developments may give a better picture of the logical grouping. I suspect the aspects of logging visits and storing pages should be more clearly separated.

[Treora]

Lots of reorganisation done in #135. It could always be better, but it seems good enough to close this.