Added clients commands to ttnctl

[egourlao]

With this PR, ttnctl integrates OAuth client management options.

• request: request an OAuth client. After this command is executed, the user receives an e-mail when his/her OAuth client is accepted or rejected. e.g.:

  ttnctl clients request oauthclient --scope "profile,gateways"`
  INFO OAuth client requested OAuthClientName=oauthclient

• edit: edit the general information (name, description...) of an existing OAuth client. e.g.:

  ttnctl clients edit oauthClient1 --name oauthclient-new-name --description "OAuth client used by the console"
  INFO Retrieving OAuth client...               OAuthClientID=oauthClient1
  INFO Retrieved OAuth client                   OAuthClientID=oauthClient1
  INFO OAuth client updated                     OAuthClientID=oauthclient-new-name

• list: list all owned OAuth clients, and shows it in the same kind of display as ttnctl applications list. e.g.:

  ttnctl clients list
  INFO Found one OAuth client:
  
      Name                    Description             URI                                                          Secret                                                          Scope                        Grants
  0       ttn-console             TTN Console             https://console.thethingsnetwork.org/oauth/callback 156a7c34fbd156d333f53b08ab6015b20b558818148a901927bb3f5b19fe    [profile apps gateways]      [authorization_code refresh_token]

• delete: delete an OAuth client

  ttnctl clients delete oauthclient1
  INFO OAuth client removed successfully OAuthClientName=oauthclient1

[htdvisser]

What happened to the decision in https://github.com/TheThingsNetwork/ttn/issues/631 ?

[coveralls]

Changes Unknown when pulling 7630ffa09d8ef204cbb40b2bf00c452c254238dc on feature/clients into on develop.

[egourlao]

I had to make this for myself to deploy the Australian region of TTN - I made register and list, so I figured that I would also do delete and edit at the same time for other use cases. A month ago, we decided to keep client request in the console - what are the current plans? Do we plan to enable the console form?

[coveralls]

Changes Unknown when pulling bb49bf98b53a38873f5dbba60c4c225ad56ed91c on feature/clients into on develop.

[coveralls]

Changes Unknown when pulling 1c89ba76fed8b7fed87204feb6474c061ab4f725 on feature/clients into on develop.

[coveralls]

Coverage remained the same at 74.883% when pulling 1c89ba76fed8b7fed87204feb6474c061ab4f725 on feature/clients into 566e30306c6d7108b71f0494ca9719fd349101d0 on develop.

[coveralls]

Coverage remained the same at 74.88% when pulling f215981fc2e19be6f76b14bfac9f3015c6459951 on feature/clients into 28a2848f7ac3fb5ac26a9c7ba7085bcabc32d8df on develop.