How should we manage the Threema CA cert?

[willnix]

The certificate is used in o3/blob.go and go-unirest/Requests.go and it would be nice to only have one instance of it, in case it changes.

this is just a reminder will add more thoughts later

[cryptix]

I'd add a GetPinnedTLSCert() []byte to o3ma/o3 and use that here and in the go-unirest fork.

Ideally this should also reduce the size of the go-unirest patches and make rebasing them easier.