This is in preperation for implementing various federation APIs that
need to build events.

The HTTP handlers in the components are split into reader and writer directories. This was a fairly arbitrary distinction, and turns out to not be so helpful. Most read APIs have a corresponding write API, and it is more natural for them to be in the same file rather than in different directories.

* Add config for trusted ID servers

* Add new error

* Implement check for trusted ID server

* Complete unfinished comment

* Make comment more explicit in the config file

* Use go standard errors in membership.go

* Use standard errors instead of JSON responses in threepid

* Doc errors

* Remove unused parameter

* Create package for handling 3pid processes and move invite processing there

* Add database table and functions for tracking 3PIDs

* Add structures and functions to interact with an ID server

* Add handlers for 3PIDs management

* Fix 3PIDs retrieval sending null if no 3PID known for a user

* Include medium in database requests and function calls

* Publish an association if it has been validated and requested

* Add TODO markers for tursted ID server check

* Use a structure instead of a map to represent a 3PID

* Structure for 3PID invite

* Generate invite from 3PID known by ID server

* Load user profile in a separate function

* Generate m.room.third_party_invite if the ID server doesn't know the 3PID

* Fix URLs to the spec in comments

* Move third-party invites to a separate package and doc' it

* Handle non-OK status codes on lookup

* Send display name to identity server when asking to store an invite

* Remove join response structure

* Change the way some variables are declared or passed as argument

* Use gomatrixserverlib.Base64String instead of the builtin base64 package

* Don't copy the public keys array

* Move events contents to common

* Basic database structure

* Complete database update

* Support visibility update and retrieval

* Add HTTP methods for visibility update and retrieval

* Add the database for the new component

* Add a listener for the new component

* Fix attribute update statements

* Create public rooms component

* Fix failing test

* Add roomserver consumer

* Fix a bug in aliases creation

* Add a check on type

* Implement public rooms directory

* Use auth API for visibility update

* Support filtering

* Add component to monolith

* Various fixes

* Fix computation of next public rooms batch

* Retrieve state events from the roomserver query API + avoid dupes on join

* Split update of string or boolean attribute in two separate functions

* Use event type to detect duplicate joins

* Improve the joined members counter computation

* Use event.RoomID()

* Implement membership endpoints

* Use FillBuilder when possible

* Fix typo in membership event content

* Fix state key invite membership events not being correctly set

* Set membership content to match the profile of the user in state_key

* Move event building and rename common function

* Doc getMembershipStateKey

* Check if user is local before lookin up their profile