Add support for static tile query string args

This application does not support non-plot map features, so we need to
prevent including dots for the non-plot map features on the map tiles.
The tiler supports this with a show= query string argument, so I have
added an OTMEnvironment setting that is appended to the tile url template.
Show layer selector on move tree view

The view already had a segmented control, but it was obscured by the nav
bar. I moved it into view and refactored the code to draw a translucent
background behind the segmented control so I could be shared by both map views.
Load instance info while on splash page

Fixes #97
Allow clearing saved keychain data

Logging out of the application would not "stick" because the keychain
wrapper did not allow clearing values.
Merge pull request #106 from jwalgran/topic/move-tree-basemap-selector

Show layer selector on move tree view
Merge pull request #103 from jwalgran/topic/hide-non-plots

Add support for static tile query string args
Handle initial errors when grabbing instance info
Merge pull request #111 from jwalgran/fix/logout

Allow clearing saved keychain data
Merge pull request #110 from OpenTreeMap/topic/load-on-splash-page

Load instance info while on splash page