Update configuration

To implement #39 we need a way to configure the prefix of the \BlueLaTeX
Rest API.
The configuration was reworked to be able to bind to any host as well.
[imp #39] Make the API prefix configurable

If \BlueLaTeX is not installed at the root of the virtual host, using
the API matchers strips the configured prefix for you, so you can write
your paths without worrying about it.
Fix restore cursor position

Forgot to count the "\n" character.
Improve new paper UI

Define default template and change <select> style
Merge pull request #70 from tdurieux/improve-new-paper-page

Improve new paper UI
Merge pull request #69 from tdurieux/fix-cursor-position

Fix restore cursor position
[fix #62] Read compilation logs in UTF-8
[imp #39] Add application configuration

The client may request the public configuration data that indicates
(amongst other things) the prefix of the Rest API, and the ReCaptcha
public key.
This object may actually contain any data needed by the client to be
successfully started.
Always catch exceptions

Exceptions that are not catch in the application are handled in the
top-level `BlueLet`.
Merge pull request #71 from satabin/wip-not-root

Wip not root
Do not fail if the paper directory does not exist

If the paper directory was not present, it was impossible to delete the
paper. This can happen for plenty of reasons, and should not lead to an
error.
Set user data on paper creation

User full name, email address and affiliation (if any) is aded in the
template.
Add contributor file
Add support for more templates

 - built-in classes article, book and report are now supported,
 - beamer template added.
Add registration captcha

Add a captcha in the registration page to prevent bot registration