UI to search signed agreements, completes auth for administrators

[RobertoPrevato]

• closes Implement admin UI to see signed CLAs #12
• closes Complete authentication and authorization strategies #8
  Admin UI to see signed CLA objects is implemented in the simplest way possible: it enables searching for CLA by contributor email address. Initially it was taken into consideration to implement a table of signed CLAs, but this would require much more work as it requires paging, etc.

Authentication for administrators work by integration with GitHub sign-in, using the same OAuth application used for contributors. CLA-bot verifies that the user who signed in is indeed an administrator, and issues a JWT. The client side verifies that an access token exists before rendering the admin UI.
APIs that manipulate information in the database require JWT Bearer Authorization header. GET methods that return read-only public information, such as agreements list and texts, are still public (they don't require an access token).

Added several other improvements to the existing code. For example, possibility to display dates in EN-GB format instead of EN-US format.