More campaign UI refactors and minor changes

Created by: sqs

• merge rendering logic for Campaign{,Plan} diffs
• merge rendering logic for Campaign{,Plan} changesets
• use auto-resizing textarea for campaign description
• encapsulate campaign plan spec form data in component
• improve placeholders for Campaign{Title,Description}Field components
• do not show username in new campaign form. It is not useful because the component does not look convincingly like a comment to make it clear that that's what we're going for.
• remove namespace field, always use current user. Right now, the namespace field in the campaign form is not useful. Site admins always see all campaigns on the campaign list page, and non-site admins can't view any campaigns at all. It adds needless complexity to the form. This commit makes all campaigns created in the current user's namespace. When we support campaign namespaces throughout the UI, we can add this field back (or something like it).
• use new ImageAutoAdjustIcon icon for campaigns. This icon better conveys "fix" and "auto" than the current icon (which is of a burndown chart going down). Also upgrades mdi-react and fixes snapshots to not depend on the particular SVG definition (which can fluctuate meaninglessly when mdi-react is updated). Example:

Created by: codecov[bot]

Codecov Report

Merging #7447 into master will decrease coverage by <.01%. The diff coverage is 89.09%.

@@            Coverage Diff             @@
##           master    #7447      +/-   ##
==========================================
- Coverage   40.46%   40.46%   -0.01%     
==========================================
  Files        1251     1253       +2     
  Lines       65331    65297      -34     
  Branches     6172     6154      -18     
==========================================
- Hits        26438    26420      -18     
+ Misses      36481    36465      -16     
  Partials     2412     2412

Impacted Files	Coverage Δ
web/src/enterprise/campaigns/detail/backend.ts	10.81% <ø> (-0.31%)
.../enterprise/campaigns/detail/diffs/FileDiffTab.tsx	85.71% <ø> (ø)
...prise/campaigns/detail/form/CampaignTitleField.tsx	66.66% <ø> (ø)
...eb/src/enterprise/campaigns/global/list/backend.ts	0% <ø> (ø)
...b/src/enterprise/campaigns/detail/CampaignTabs.tsx	100% <100%> (ø)
...campaigns/detail/changesets/CampaignChangesets.tsx	100% <100%> (ø)
web/src/enterprise/campaigns/icons.ts	100% <100%> (ø)
...nterprise/campaigns/detail/diffs/CampaignDiffs.tsx	100% <100%> (ø)
...campaigns/detail/form/CampaignDescriptionField.tsx	83.33% <75%> (+8.33%)
...rc/enterprise/campaigns/detail/CampaignDetails.tsx	62.08% <83.33%> (-0.93%)
... and 6 more

Created by: sqs

Sounds good. I’ll make that change tonight in about 18 hours from now. Want to approve it otherwise?

Created by: sqs

If you could do it, that would be great. Thanks!