Postgres support for uadmin + bunch of minor fixes.

For the last few days:

1. added support for postgres
2. small code improvements
3. check user permissions now before opening any admin page
4. updated documentation, added section: contributing: https://uadmindocs.sergeyg.me/docs/contribution
5. mentioned in the docs how to use hot reloading of your go app on file changes
6. added badges that related to new uadmin: ci-cd jobs, godoc, test coverage, license
7. fixed 500 error when session expired
8. added command: dbshell (opens database shell) for your database
9. added command: shell which opens go interactive shell for you.
10. humanize field names in admin panel for you.
11. use html/template instead of text/template
12. fixed some 404 errors in admin panel
    13, added few middlewares to check for the session issues.
13. added ci-cd jobs.