static URLs to "Current Version" of each app

I wrote up the feature to automatically generate symlinks with a constant name
that points to the current release version. I have it on by default, with a
*config.py* option to turn it off. There is also an option to set where the
symlink name comes from which defaults to app['Name'] i.e. F-Droid.apk, but
can easily be set to app['id'], i.e. _org.fdroid.fdroid.apk_. I think the best
place for the symlinks is in the root of the repo, so like
https://f-droid.org/F-Droid.apk or https://guardianproject.info/fdroid/ChatSecure.apk

For the case of the current FDroid static link https://f-droid.org/FDroid.apk
it can just be a symlink to the generated one (https://f-droid.org/F-Droid.apk
or https://f-droid.org/org.fdroid.fdroid.apk). Right now, this feature is all
or nothing, meaning it generates symlinks for all apps in the repo, or none. I
can’t think of any problems that this might cause since its only symlinks, so
the amount of disk space is tiny. Also, I think it would be useful for having
an easy “Download this app” button on each app’s page on the “Browse” view. As
long as this button is less prominent than the “Download F-Droid” button, and
it is clear that it is better to use the FDroid app than doing direct
downloads. For the f-droid.org repo, the symlinks should probably be based on
app['id'] to prevent name conflicts.

more info here:
https://f-droid.org/forums/topic/static-urls-to-current-version-of-each-app/
when symlinking current version of app, include gpg sig if it exists

Let's make it easy for people to also find the GPG signature, and promote
easy verification!
remove url-unsafe characters from "current version" symlink names

This prevents the URL from having ugly %20 stuff in the app name.
server: upload "current version" symlinks if requested

If `fdroid update` generates the "current version" symlinks, then
`fdroid server update` should upload them to the server.
server: always use same rsync destination path to allow for strict setups

for more info on how:
http://positon.org/rsync-command-restriction-over-ssh
http://ramblings.narrabilis.com/using-rsync-with-ssh
Merge branch 'current-version-links' into 'master'

Current version links

This is the automatic generation of "current version" symlinks that @ciarang and I chatted about, and is discussed here: https://f-droid.org/forums/topic/static-urls-to-current-version-of-each-app/

See merge request !29