This PR fixes #7598 by providing a configurable way of signing commits across the Gitea instance. Per repository configurability and import/generation of trusted secure keys is not provided by this PR - from a security PoV that's probably impossible to do properly. Similarly web-signing, that is asking the user to sign something, is not implemented - this could be done at a later stage however.

## Features
- [x] If commit.gpgsign is set in .gitconfig sign commits and files created through repofiles. (merges should already have been signed.)
- [x] Verify commits signed with the default gpg as valid
- [x] Signer, Committer and Author can all be different
    - [x] Allow signer to be arbitrarily different - We still require the key to have an activated email on Gitea. A more complete implementation would be to use a keyserver and mark external-or-unactivated with an "unknown" trust level icon.
- [x] Add a signing-key.gpg endpoint to get the default gpg pub key if available
    - Rather than add a fake web-flow user I've added this as an endpoint on /api/v1/signing-key.gpg
    - [x] Try to match the default key with a user on gitea - this is done at verification time
- [x] Make things configurable?
    - app.ini configuration done
    - [x] when checking commits are signed need to check if they're actually verifiable too
- [x] Add documentation

I have decided that adjusting the docker to create a default gpg key is not the correct thing to do and therefore have not implemented this.

Signed-off-by: Tamal Saha <tamal@appscode.com>

On merge we walk the merge history and ensure that all lfs objects pointed to in
the history are added to the base repository. This switches from relying on having git-lfs installed on the server, (and in fact .gitattributes being correctly installed.)

* Ensure that the lfs files are created with a different prefix
* Reduce the replication in git_test.go

* move sdk structs to moduels/structs

* fix tests

* fix fmt

* fix swagger

* fix vendor

1. A key can either be an ssh user key or a deploy key. It cannot be both.
2. If a key is a user key - it can only be associated with one user.
3. If a key is a deploy key - it can be used in multiple repositories and the permissions it has on those repositories can be different.
4. If a repository is deleted, its deploy keys must be deleted too.

We currently don't enforce any of this and multiple repositories access with different permissions doesn't work at all. This PR enforces the following constraints:

- [x] You should not be able to add the same user key as another user
- [x] You should not be able to add a ssh user key which is being used as a deploy key
- [x] You should not be able to add a ssh deploy key which is being used as a user key
- [x] If you add an ssh deploy key to another repository you should be able to use it in different modes without losing the ability to use it in the other mode.
- [x] If you delete a repository you must delete all its deploy keys.

Fix #1357