Support for designating themes for activities or sequences

[#52741243] Adds the 'theme' field to the Sequence and LightweightActivities models, along with form fields to set or change it and tests to confirm those controls are present. The field is stored as a string, but we're using a select menu to pick from a fixed array of strings; that array is set as a constant in the initializer config/initializers/themes.rb .

https://www.pivotaltracker.com/story/show/52741243
Suppresses a warning in development
WIP on Theme object: 95% of this model
Finish themes testing
Fixes thème index listing
Using Theme objects instead of strings for Sequences and LightweightActivities

This is a slightly different implementation of [#52741243] to use the new Theme object instead of just a string.

https://www.pivotaltracker.com/story/show/52741243
Rename 'upstatement' layout to 'runtime'
First themes (HAS, MW) and CSS organization

[#52741509]

https://www.pivotaltracker.com/story/show/52741509
Oops - didn't define destroy
Move footer into new Project model

For [#52056969] and [#52741509] - the logo, URL and project title (and, logically, footer) should belong to a Project object, not the theme. This creates the Project and moves the right parts of Theme to that object.

https://www.pivotaltracker.com/story/show/52056969
Associate Sequences and Activities with a Project
Fixing up Projects controller
Fixing up Project admin
Fixing theme/project assignment
Assigning @project and @theme when we need them

[#52741509] and [#52056969]

https://www.pivotaltracker.com/story/show/52056969
https://www.pivotaltracker.com/story/show/52741509
Use @project and @theme for header, footer and CSS

Note that this changes the landscape for [#51730923] because the link in the upper left is now that of the project, and the logo is as well. It's still the case that it's unclear how to get to the LARA index page, but I'm not convinced we need to get to the LARA index page from the runtime.

[#52741509] [#52056969]

https://www.pivotaltracker.com/story/show/52741509
https://www.pivotaltracker.com/story/show/52056969
https://www.pivotaltracker.com/story/show/51730923
Merge branch 'refs/heads/master' into theming
Restore link to activity TOC in navigation [#52734761]

https://www.pivotaltracker.com/story/show/52734761