Redirect traffic to a page w/o db access during backup write/restore

[BenMusch]

Fixes #284

Should help make backups a bit safer

[codecov-io]

Codecov Report

Merging #285 into master will increase coverage by 0.14%.
The diff coverage is 84.37%.

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   61.33%   61.47%   +0.14%     
==========================================
  Files          63       63              
  Lines        4236     4257      +21     
==========================================
+ Hits         2598     2617      +19     
- Misses       1638     1640       +2

Impacted Files	Coverage Δ
mittab/settings.py	95.83% <ø> (ø)	⬆️
mittab/libs/backup/__init__.py	72% <82.6%> (+6.21%)	⬆️
mittab/apps/tab/middleware.py	88% <88.88%> (+0.5%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update daa91af...f289baf. Read the comment docs.