[Feature] 3pid invite

As specced [here](https://matrix.org/docs/spec/client_server/latest#third-party-invites)

The app should:
- [x] Render the `m.room.third_party_invite` event in the timeline

To send invite, the app should:

- [x] Send invite to an email with `/invite` command
- [ ] Send invite to a Msisdn with `/invite` command -> We get an error 500 {"errcode":"M_UNKNOWN","error":"Internal server error"}
- [x] In the room member list, access the phone contacts to be able to invite them
- [x] by email
- [ ] by phone
- [x] by Matrix ID thanks to identity server lookup

Also:

- [x] If power level is high enough, user should be able to revoke a 3pid invite

When receiving an invite, the user click on the link and the app should:

- [ ] If the app is not installed: offer to install the app (redirect to the store?). Currently I think the riot.im web instance will be opened. The mail should propose to install the app.
- [ ] If the user is not logged in: the user should be able to create an account or to log in and see the invite once he has bound his 3pid to the account
- [ ] If the user is already logged in: the user should be able to see the invite if the 3pid is bound to his account

EDIT: First PR will handle all the checkbox above. Msisdn support is deactivated for the moment