Support pgp trust levels

[danielemery]

PGP keys use a trust model, with levels such as Ultimate, Trusted, Untrusted etc.

If they keys project will be used as a method to store trusted keys as well your own keys, the concept of trust should be considered.

It's proposed that within the keys folder there can be optional subfolders, one for each trust level.

• If at least one of these exist keys in the root folder will be considered Untrusted otherwise keys will be considered Ultimate
• Keys in the subfolders will be treated with the trust level of the folder
• The /pgp endpoint will be updated to allow a query argument for ?trust to allow filtering on trust

[danielemery]

Since #51 there was a pivot to use the config file to mount pgp keys. This means:

• This issue would no longer be considered a breaking change (so we will remove the v2 milestone)
• We can simply use new optional fields in the config file to specify trust levels