Fix #1803: Constraint runtime errors.

[iccub]

Summary of Changes

This fixes some constraint problems in:

• bvc(there's still one more runtime error I haven't managed to fix)
• search controller
• shields view controller

There's one error I couldn't fix, shows up when keybard is shown for the first time

This pull request fixes issue #1803

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

This is for reviewer: run the app and look at logs if there's still some constraint errors.

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).

[Brandon-T]

Hmm, is there a reason we constrain the scrollView in layoutSubviews?
Just wondering if this will trigger another layout.

I tested it and it doesn't seem to trigger anything so looks good to me.

[Brandon-T]

This works. It will also work if constrained to the contentLayoutGuide.

IE: scrollView.frameLayoutGuide.constrain(to: self) and stackView.constrain(to: scrollView.contentLayoutGuide).

But your code works so it's fine :)

[kylehickinson]

@iccub Please rebase :)