Print FROM and TO SHA1's during upgrade

Useful if someone pastes an unexpected upgrade.py log into a bug report
(also nice for people who are not familiar with git reflog).
Introduce systemidentities.{installed_sha1,installed_at} database columns

After installation or upgrade, the installed_sha1 column will contain the git
SHA1 of Critic which is running that identity and the installed_at column
will contain a timestamp for the last install/upgrade.
Show installed Critic version in the frontend
Include time of Critic installation in review ETag

Before this fix, if /r/1 was created from branch "r/user/11111" containing
commit A, and then Critic was reinstalled, and the same was done again except
with commit B instead; then browsers would show the old outdated /r/1
(chromium even needed a clean SHIFT-CTRL-R reload to show the right content,
inflicting pain in digitus minimus manus).
Fix JS syntax error for anonymous users loading a review page
Make --is-development set configuration.base.IS_DEVELOPMENT to True in install.py