Hide view writeup button when no writeup exists

[damianhxy]

Description

• Hide view writeup button when writeup does not exist
• Use flash instead of text to report when writeup does not exist

Note: Defining an analogous check for handouts is harder because the config file might override the default handout behavior

Also note: flash.now is necessary because we are rendering directly rather than redirecting (as otherwise the flash would persist for an additional page load). I have updated handout's code to use flash.now, but it appears that flash works too because the route is defined in a different file and that somehow makes a difference.

Motivation and Context

Currently, the view writeup button is visible to students even if no writeup exists. Furthermore, the error text is unnecessarily large.

How Has This Been Tested?

Before

Error Text

Button visible

After

Error Flash

Button not visible

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting

[20wildmanj]

• verified writeup option disappears from menu when none specified
• verified writeup button stills works when specified
• verified new /writeup page modifications
  LGTM