INSTALL.md is inconsistent in its treatment of optional dependencies

[richvdh]

On the one hand, INSTALL.md tells you to install synapse with pip install matrix-synapse[all]. On the other hand, it lists prerequisites assuming that you are just installing the base functionality - and even mentions libxml2-dev under the 'URL previews' section, when in reality you couldn't have got that far without installing it.

The 'optional dependencies' feature doesn't seem to have a great deal of value any more, since we always tell people to install with [all]. Perhaps we should just make the optional dependencies mandatory, and if you don't have a libpq-dev or a libxml-dev on your platform, sucks to be you?

[richvdh]

PRs blocked by a decision on this include #5082, #5106.

[richvdh]

This has been much exacerbated since the release of psycopg 2.8, which requires libpq-dev, so we need to reach a decision on this.

[richvdh]

Further to discussion among the team today: the optional dependencies should be optional. We may need to reconsider exactly which dependencies are optional, but the solution to that is to do so, not to tell everybody to install everything.