Add CDN logic, mTLS ability for CDNs, and Branding for webpage

[codeyschoettle]

This PR makes several breaking changes to the base code.

• Adds the ability to use CDN URLs in the repositories list. This is crucial if RHEL repos need to be used via RedHat's CDN.
• Adds the ability to utilize mTLS for requests. This is required when attempting to use RHEL repos.
• Adds a "branding" section to the config file. This is to give the landing page a bit more flexibility when a dev utilizes it in their own environment. This also adds a display of the pkgproxy version currently being served.
• Changes the way the URL is injected for code snippets on the landing page. This was added as a means to serve pkgproxy behind a reverse proxy where the URL would be different. Added JS to pull the origin URL when the page is being displayed.