[minor] improve 403 (forbidden) HTML page

[jameshadfield]

Current Behavior

Accessing a private group (or dataset within the private group) when when logged in but without access to said group currently returns a 403 with text/html with content "Forbidden"

Expected behavior

The page should be the similar to when attempting to access a non-existent group (e.g. what happens at https://nextstrain.org/groups/nothing-here) but with the banner text explaining that your account does not grant you access to this group. (Some websites choose to return a 404 in this case, but we have historically taken a different approach.)

Note: accessing the page when not logged in returns a login prompt, as expected.

[jameshadfield]

This is essentially #774 but for a different (server-side) HTTP error