Cache all 'getAllPossibleTokens' on startup

[nico-ptrs]

Expected behaviour

When a user wants to fund something, he (nearly) immediately wants to see the tokens he can use.

Actual behaviour

When a user goes through the funding wizard and it is the first time an issue for that owner/repo gets funded, the allowed tokens are fetched from the tokenWhitelistPreconditionContract, but this can take a while (a couple of seconds).

Possible sollution

Fill the cache on startup by calling the 'getAllPossibleTokens' for every owner/repo for which we have requests