[ui] show warning about saving docs changes in reports

The documentation editor for BEAT objects reloads the page
when the user clicks "Save". This reload discards unsaved
changes in the reports app. Unfortunately the documentation
system is object-independent, so the entire system can't be
changed just for the reports app.

Instead, a warning is shown when editing the documentation
in a report.

Closes #528
1 job for <span class="ref-name">528-doc-report-reset</span> in 21 minutes and 18 seconds (queued for 9 seconds)
Status Job ID Name Coverage
failed #167897


Name Stage Failure
build_linux_36 Build
TOTAL 12739 6354 50%
Ran 154 tests in 29.622s

FAILED (errors=2)
Coverage.py warning: Module beat.cmdline was previously imported, but not measured (module-not-measured)
Coverage.py warning: No data was collected. (no-data-collected)
ERROR: Job failed: exit status 1