Move login button and highlight no login required for downloading

[Xpirix]

So here is how we propose to address this:

1. We will move the login to the other side of the submenu, after the other submenu items

2. We will redesign the login screen to emphasis that you DO NOT NEED TO LOG IN in order to download plugins. In fact @Xpirix we should be directing users to use the plugin manager in QGIS as the first choice if they simply want to use the plugin in QGIS. We will change the background so that this information is in high contrast and above the login area so the user is more likely to read it.

3. Medium term: We will phase out direct LDAP autthenacation to rather authenticate via our own keycloak server with OSGEO LDAP being one of the providers behind keycload but also offering a range of other social auth providers they can use. That should radically reduce the amount of requests you get.

Originally posted by @timlinux in #225 (comment)