feat: always use json for http requests/responses

[gruyaume]

Description

Here we ensure that every API calls takes json objects as request parameters and return json objects in its responses.

Standardized responses

Each return object now has the following format

{
  "result": "...",
  "error": "...",
}

Rationale

Request parameters

HTTP Request Parameters were inconsistents. Some had to be json formatted, some didn't. For example creating a user required a the following body:

{"username":"admin", "password": "Admin1234"}

And creating a certificate required the bare certificate:

-----BEGIN CERTIFICATE REQUEST-----
MIICsTCCAZkCAQAwbDELMAkGA1UEBhMCQ0ExFDASBgNVBAgMC05vdmEgU2NvdGlh
MRAwDgYDVQQHDAdIYWxpZmF4MSEwHwYDVQQKDBhJbnRlcm5ldCBXaWRnaXRzIFB0
...

Related quality improvements

Doing so we address many of related Ben's feedback. Namely:

• Centralize HTTP responses into writeError and writeJSON functions
• Create separate structs for DB and API objects - Never return db structs
• Remove tests that wrote to the developer's home directory
• Rename HTTP handler's files so that they match their endpoint
  • handlers_health -> handlers_status (endpoint: /status)
  • handlers_users -> handlers_accounts (endpoint: /api/v1/accounts)
• Move test certificates to files under testdata instead of having long strings in test files
• Use numUsers to get the number of users instead of listing them and computing the list length

This is a breaking change

This is a breaking change, anything that depended on Notary's API (here luckily only the charm) will need to be adapted.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[kayra1]

Thanks