fetch ALLOWED_USERS from github team

[mordamax]

it doesn't make sense to store ALLOWED_USERS in vault because:

• we anyway output the list of users who should approve it
• we don't have visibility and open control for allowed users within org, if some user leaves company or becomes team lead

In order to simplify managing of allowed users - lets move it under the github team control

• create a team in GH (something like tip bot approvers or similar)
• add existing allowed users (as long as they are in ParityTech org) - shawntabrizi, gavofyork, rphmeier, athei, andresilva, arkpar, bkchr, eskimor, drahnr, dvdplm, robbepop, cmichi, tomaka, pepyakin, kianenigma, rossbulat
• implement dynamic check of allowed user using github api

[mutantcornholio]

The team: https://github.com/orgs/paritytech/teams/tip-bot-approvers/members

People, who aren't in @paritytech right now, so not in the list: drahnr, tomaka.

Now, to the question of who should maintain the team. Feels like we (meaning @paritytech/opstooling) shouldn't be in the team, so probably not us, right?

cc/ @mordamax @lovelaced

[mordamax]

@mutantcornholio we can be in the list, so we can also add/remove people, since we own the bot.
who else can control - not sure. I'd make everyone to be able to add more people there. Should be safe, cuz the tip will anyway be reviewed separately later here https://polkadot.js.org/apps/?rpc=wss%3A%2F%2F1rpc.io%2Fdot#/treasury/tips

[mutantcornholio]

done