Handle trailing and leading slashes in config values

[ericlarssen-wf]

Adding a path such as /assets to ASSETREV_BASE_PATH will result in a path of /static//assets.

[WilliamMayor]

Similarly to this, my manifest.json file contains absolute URLs, and this caused paths to contain /static//xys.js.

I got round this by setting the ASSETREV_BASE_URL to not have a trailing slash. This means I have to hardcode my base URL for each environment (but that's not too arduous).

[ericlarssen-wf]

😦