Trailing / in repo URL is not stripped

[rgaiacs]

This was first reported by @JuKo007.

Bug description

I believe that in the the old UI, BinderHub was stripping the leading / in the URL provided by the user. For example, if the user provides

https://github.com/gesiscss/Jupyter4NFDI_survey_results/

the UI would process as

https://github.com/gesiscss/Jupyter4NFDI_survey_results

How to reproduce

1. Go to https://notebooks.gesis.org/ or https://2i2c.mybinder.org/
2. Fill "URL" with https://github.com/gesiscss/Jupyter4NFDI_survey_results/

Expected behaviour

Launch URL to be

https://2i2c.mybinder.org/v2/gh/gesiscss/Jupyter4NFDI_survey_results/HEAD

Actual behaviour

Launch URL is

https://2i2c.mybinder.org/v2/gh/gesiscss/Jupyter4NFDI_survey_results//HEAD

Notice the //. This is making BinderHub to not be able to resolve the reference.

[rgaiacs]

This is part of #1920.