More flexible search when inviting someone to a note

From https://github.com/hackmdio/hackmd-io-issues/issues/127#issuecomment-551009697.

**Is your feature request related to a problem? Please describe.**

When inviting someone to a note (following https://hackmd.io/c/tutorials/%2Fs%2Finvite), it is sometimes hard to find them. For example, when the person use the github authentication, you have to search `FirstName LastName` rather than `LastName` or than `github-username` or any substring of the username. As an example, if I type "grisel" in the search bar I should be able to find "Olivier Grisel"
and I am not (see screenshots below):



**Describe the solution you'd like**

Below are another few nice things to have (in an ideal world, I completely understand you may have higher priorities). Not sure how hard they are technically (probably not that straightforward) and whether some are easier than others:
- using "hackmd.io username" could be specified in the tutorial (https://hackmd.io/c/tutorials/%2Fs%2Finvite). The search bar has a text (before you type anything) saying "search user by email ..." maybe it "or hackmd.io user name" could be added (if it fits ...)
- search by github (Twitter, or other auth types) names. By the way, I am not sure what username I get when I sign in with github. Maybe you should keep the github username (e.g. lesteve) rather than my full name (e.g. Loïc Estève)
- more flexible search: if I type "grisel" in the search bar I should be able to find "Olivier Grisel"