Add "preferences" panel to web client

[JeanMertz]

Building on top of #11, the right-most icon should be a gear icon that opens the preferences panel.

For starters, it should be possible to "log out" from within this window, which removes active cookies. This allows you to switch to a different session if need be.

More options will show up here as we go.

[JeanMertz]

Copy paste from my (old) local TODO list:

• Add some kind of preferences window in the web-client, to configure some
  things. This could probably be tagged onto the bar shown in
  Add filters to task list #11, with some kind of gear
  icon to open the preferences. Some things to add: manage
  favorites/one-clicks, toggle single-column vs multi-column results (so
  that you can have more pipelines on screen, for power users), perhaps a
  dark-mode toggle, perhaps a "default search query" preference when
  opening the app, a "hide statistics" toggle for more screen real estate

• Complimenting the above, we could start storing this information
  server-side using some kind of "unique user token" instead of a complex
  login system. We could then start exposing "feature flags" that allow
  tweaking the experience. These feature flags can be either server-side
  toggles, or client-side, but the client doesn't have to care, it just
  exposes these feature flags (or a subset of what's implemented in the
  client) in the preferences window. For example show_statistics is a
  feature, or multi_column_results, etc