Fixes several crashes

[venkatesh-sivaraman]

Full list of crashes fixed:

• Creating a new document caused a main thread networking error
• Crash when substituting with a custom course and then trying to edit the substitution (one error fixed, but it still crashes - more work needed)
• Trying to sync progress assertions using old progress overrides caused a crash
• Showing dialogs when activity/fragment is not visible caused a crash

Also fixes an issue where subjects could get deleted from the road if a database update was started but not finished (e.g. due to a crash or the user leaving the app). This is likely what was causing a lot of the lost road contents. The new behavior is to ensure that the database update restarts if it didn't complete on the last run, and to show all subjects from the road file even if they're not present in the db.

[venkatesh-sivaraman]

Looks good to me. @liuric could you verify that the bugs I listed above no longer occur? If they're all good then we should send out the update ASAP.