Display multiple mechanisms

[stulacy]

This PR adds the ability to view multiple mechanisms in the MCM website. The first commit has added the ability to select mechanism in the backend through the route (i.e. all routes are now sublevels of a /<mechanism/ route).

The outstanding features are:

• [x] In any backend route that interacts with the reactions in the DB, subset to the currently viewed mechanism:
  • [x] /browse
  • [x] /species
  • [x] /search
  • [x] /export
• [x] Add a UI element for toggling between mechanisms
• [x] Test export of both mechanisms against current site
• [ ] Add ability to view old MCM versions (not necessary but worth doing if have time)

[stulacy]

06acdee added a UI element visible on every page to switch between mechanisms. It needs a bit of visual polish and testing that exports from both mechanisms are consistent with the old website before release.