Allow installing an app "under" the subpath of another

[Salamandar]

A use case :

• install landingpage on /
• install grav for doc on /doc

We could have an app setting allow_app_under_path or something equivalent to declare that another app can be installed under it.

[zamentur]

For now it's totally forbidden to install an app on a subpath of another.I guess we could decide to allow it after making a curl check on the subpath. If it's 403, the subpath is already in use, if it's a 404, we could tolerate the choice of the admin... BUT, this approach has some limitations:

• some app could suddenly use the subpath after the sub app has been installed. Could be due to an upgrade or due to a user action (e.g an editor on a CMS could suddenly use a subpath that was not in use before)
• the root app could have a robots.txt or a sitemap in conflict with "the sub app"
• the sub app could access to cookies of the root app, and be a way to attack the whole things
• the sub app and the root app could have distinct sso header needs, so the sso should be checked to be sure it can support this kind of config