Rework commit page header (#17849)

- Remove blue background and use regular header styling
- Use flexbox on author and signature segments
- Add shield-type icons besides gpg key, replacing icon font icons
- Ensure author and signature line are same height
- Remove erronous green background on arc-green signature line
- Turn signing colors into CSS variables
- Consolidate the two warning states into one

Co-authored-by: zeripath <art27@cantab.net>

web_src/less/helpers.less

@@ -2,8 +2,10 @@
 .di { display: inline !important; }
 .dif { display: inline-flex !important; }
 .dib { display: inline-block !important; }
+.pr { position: relative !important; }
 .ac { align-items: center !important; }
 .tc { text-align: center !important; }
+.tl { text-align: left !important; }
 .jc { justify-content: center !important; }
 .js { justify-content: flex-start !important; }
 .je { justify-content: flex-end !important; }