Enhancement/Enable token authorization for API calls

[c-w]

As discussed in #299, there is desire to interact with Doccano via the API since this enables integrating Doccano with others systems, e.g. fetch data from system A, push it into Doccano, and later sync the labels to system B.

In order to facilitate usage of Doccano via the API, this pull request adds token authentication to the API:

# fetch an auth token once for non-interactive usage of the API
curl 'http://localhost:8000/v1/auth-token' -d '{"username": "user", "password": "pass"}' -H 'Content-Type: application/json'

# at a later point, can use the auth token to interact with the API
curl 'http://localhost:8000/v1/me' -H 'Authorization: Token xxxxxxxxx'

[kechan]

I tried:

curl 'http://localhost:8000/v1/auth-token' -d '{"username": "user", "password": "pass"}' -H 'Content-Type: application/json'

But failed, among the message:

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