Configure registration validation on server side

As part of #98, \BlueLaTeX server administrator can choose how newly
registered users validate their registration.
[fix #175] Fix configuration keys in client

The client must read the correct configuration keys for clone and issue
URLs, which are respectively `clone_url` and `issues_url` separated with
underscores.
Moreover, the reCAPTCHA element must be imported wherever it is used.
[imp #168] Reload pdf on non critical errors

Sometimes, the compilation of a document with \LaTeX produces some
errors, but a pdf file is still generated.
In such cases, the web client now loads the new pdf file, while still
indicating errors.
If no pdf file was produced, nothing changes.
Merge pull request #177 from satabin/feature/update-pdf-on-error

[imp #168] Reload pdf on non critical errors
[imp #173] Add button to cleanup files

Sometimes, we need to explicitly clean files in the compilation
directory. The server has supported this for a long time now, without
being exposed in the web client.
That is now the case.

I also used this chance to improve the recompilation after a cleanup. If
the .aux file cannot be found, the compilation is performed, even if the
paper did not change. This makes sense, because if the user cleaned up
the compilation directory, it probably means that he wants a fresh
compilation to be performed.
Merge pull request #176 from satabin/fix/right-configuration-key

[fix #175] Fix configuration keys in client
Merge pull request #179 from satabin/feature/cleanup-compilation-files

[imp #173] Add button to cleanup files
Include fixes from v1.0.x
Update changelog
Propose password input when registering

Depending on the configuration returned by the server, the client
decides whether to require registering users to input their password in
the registration form.

This is part of resolution of #98
Merge pull request #174 from satabin/feature/configurable-email-validation

Feature/configurable email validation