Allow specifying application root path via the environment

[CyberShadow]

LineageOTA needs to know the full external URL to itself in order to generate correct links in the JSON emitted by the /api endpoint.

In typical circumstances, no manual configuration is necessary - it is able to infer it from the URL that it is accessed by, or from the Forwarded header, or from the ro.build.ota.url property from .prop files. However, when it is running behind a reverse proxy (and in the absence of the .ota.url property) , the URL prefix is not available to LineageOTA, so unless it matches the prefix on the local webserver, it is unable to construct a correct URL to itself in this scenario.

To allow it to do so, a mechanism must be added to provide this information to it. There are two straight-forward approaches:

• Provide this information externally, i.e. using a configuration file or an environment variable

• Provide this information in an HTTP header, in the same manner as the Forwarded header.

In this pull request, the former method is implemented. The rationale for not using the latter method is that administrators would need to be aware of this change and override or block the header in their reverse proxy configuration, to prevent it being forwarded (and thus spoofed).

Fixes #75.

[julianxhokaxhiu]

Thank you :)