Update to new access tokens url pattern

[dabutvin]

See https://developer.github.com/changes/2018-08-16-renaming-and-deprecation-of-github-app-installation-access-token-route/

[dabutvin]

We've been using https://api.github.com/installations/:installation_id/access_tokens everywhere

now we need to update to https://api.github.com/app/installations/:installation_id/access_tokens

There are some places where we are reading and writing accessToken URL from the InstallationsTable. We should stop doing this altogether, it seemed like a good idea at the time, but we should just use the installationId and the access token pattern everywhere and stop reading and writing this data.

For example, we should remove these properties https://github.com/dabutvin/ImgBot/blob/8f893b3184a920bcf61e707c0df818a9089d1f7d/Common/Messages/CompressImagesMessage.cs#L5
https://github.com/dabutvin/ImgBot/blob/8f893b3184a920bcf61e707c0df818a9089d1f7d/Common/Messages/RouterMessage.cs#L9