Feat: tag model and its API

[haotianzhu]

Check List

• [x] Fix #. (write the relative issue number)

• [x] Branch is up to date with master

• [x] Now it's a good time for reviewing

• [x] Branch should have prefix either "feature/" or "fix/"

Comment

Put 'closes #.' in your comment to auto-close the issue that your PR fixes (if such).

[haotianzhu]

API: GET /tags POST /tags PATCH /tags/:id delete /tags/:id GET /tags/:id/questions Tag has a unique field name. I keep id in case that you want to allow special characters for tag's name. The id is only for URL, you don't need to worry about id at request's body.

[haotianzhu]

For reviewing purpose: This is the TAG model

class Tag(models.Model):
    '''
    name: the name of tag
    '''

    class Meta:
        app_label = 'polls'

    name = models.CharField(unique=True, max_length=20, null=False, blank=False)

    def __str__(self):
        return self.name

API Part, click this

[TianqiCS]

related: #26