Fix empty telescope path config in frontend

[mmachatschek]

This PR fixes the frontend, which would break if the config('telescope.path') would be set to either empty or '/'.

Additionally, the configuration for config('telescope.path') and config('telescope.domain') now supports the possibility to overwrite the values via the .env file.

There are no frontend tests, so unfortunately I can not provide any tests. I click tested as good as I could.

Benefits

For environments (local and production) where you have a custom subdomain for telescope e.g. telescope.example.local you can now omit the prefix of the telescope routes by setting the .env variable option TELESCOPE_PATH to either empty or "/"

This makes telescope available via the following route http://telescope.example.local/ instead of http://telescope.example.local/telescope

[taylorotwell]

@themsaid can you review this?

[themsaid]

thanks :)

[mmachatschek]

@themsaid You're welcome