Allow custom ports in Pullrequest URL

My workplace uses a custom gitea installation that requires an url like `:`. The configuration of the pullrequest service does not allow me to specify this port.

**Describe the solution you'd like**
I would love to be able to specifiy ```
'git@': 'gitea::'

**Describe alternatives you've considered**
I am not sure if it is possible to remove the custom port requirement for this repository. The default port is currently still in use by our old svn repository.

**Additional context**
The error message when running `lazygit -d` is `Unexpected format for git service: 'gitea::'. Expected something like 'github.com:github.com'`.
As soon as I remove the part `:` from the configuration it opens my default browser to the new pullrequest, which does not work as the port specifier is missing.

I think this could also be considered as bug, but as far as I understood this situation was not discussed, while implementing the feature to open pull requests from within lazygit.