add env map to error page

[EriksonBahr]

Description

Inject env vars to error page so custom (and enhanced error pages) can be created. So these envs can be accessed via .Env.MyEnv, the initial intent is to add arbitrary URLs that the user can be redirected to (instead of the default Redirect urls).

Motivation and Context

The default error page looks like a bootstrap develop page and we want to make it more like a philips one. See figma below

How Has This Been Tested?

Added a custom error page to our own docker image via

COPY ./static_pages/ ./static_pages

ENV OAUTH2_PROXY_CUSTOM_TEMPLATES_DIR=./static_pages

Then authenticated in our app, then press the back button in the browser and then refresh the page:

Checklist:

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.