Store web URL for repo on src.origin and make it customizable

As part of computing the origin info, the web URL is derived from the git repo by removing the trailing .git suffix. This value can then referenced in the edit URL pattern. This value should also be made available via the webUrl property on src.origin. For example:

• src.origin.url = 'https://gitlab.com/antora/antora.git'
• src.origin.webUrl = 'https://gitlab.com/antora/antora'

In addition to storing the web URL, it should be possible to specify a custom value in the playbook. This value will then be used in place of the computed value. It can also be referenced in the edit URL pattern.

content:
  sources:
  - url: https://githost/org/repo.git
    web_url: https://viewer-githost/repo-server/org/repo