Do not check weboob install/version at each request on /all

At the moment, each time we make a /all request, we call weboob to check that it's installed and to get its version. This call might take 1 second for me (on a total of 3 seconds).

We should instead:

• When launching Kresus, define weboob-version to 0 and try to retrieve the actual version by calling weboob.
• Remove weboob-installed : checking that weboob-version is >= than the minimum version is enough
• For each request to all, if the version cached on the server is < minimum, we still request the version (by calling weboob), else we send it as it is.
• When the client settings view is displayed, we fetch the weboob version and force the request to be sure the version is uptodate, and of course overwrite the cached value. During the fetch, a small spinner is displayed in place of the version number.