Use protocol-neutral javascript references

This fixes Mixed Content problems when serving via https.
Add 'switch' to user list; 'switch back' to user nav
Add grade level filtering to the activity search page
Add subject area filtering to the activity search page
Add sensor type filtering to the activity search page
Visually clean up the new search filtering options
Change the snapshot disabled messages
Deploy task for clearing the fragment cache
Change "already have an account" wording
Only show sensors which have a prototype data collector in the activity search filters
Fix pagination result count when using group by
Require manual action to start loading a model
Don't set the interactive iframe src initially

... since it will still load, even though it's not showing.
Use IFramePhone to tell interactive models to stop whenever another interactive is played

That way we only ever have one model running at a time.
Add common instructions to labbook buttons in browser-based activities
Don't show click to play messages by default

Interactives that can run will enable them.
Add "how did you hear about itsi" back to teacher signup
Merge branch 'itsisu-dev' into itsisu-staging