General fixes & improvements

[Oaphi]

closes #1968 (the label is now configurable via locale strings):

closes #1971:

closes #1975 - comment threads now use the priority_order common scope regardless of whether they are initially loaded or expanded via the "show more" button (note that we still order by updated_at and not last_activity_at - we should switch to the latter when we can).

closes #1970:

closes #1965 (see below);
closes #1966 (see below);
closes #1884 (we now simply use Moment.js to format timestamps - note the new format and preserved relative time):

closes #1977 (also exposes MaxUploadSize to client-side code as QPixel.MAX_UPLOAD_SIZE):

closes #1969 (with existing threads and no threads respectively):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.96%. Comparing base (fb4d5da) to head (d640db3).

Additional details and impacted files

Components	Coverage Δ
controllers	75.57% <ø> (ø)
helpers	85.05% <ø> (ø)
jobs	81.48% <ø> (ø)
models	93.03% <100.00%> (-0.01%)	⬇️
tasks	61.11% <ø> (ø)

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[cellio]

Tested:

• comment label (1968)
• search width (1971)
• thread order (1975)
• profile stats alignment (1970)