Browsable api PATCH method support

I've reimplemented the PATCH method support in browsable api. I've added tabs for normal object form and for generic content type. The first one has only the POST button, the second one has PUT and PATCH button (of course if both PUT and PATCH methods are available).

I'm also not sure, if the same tabs feature is required on the POST form.

BTW, what do you think of reducing html code by adding name and value attributes to button? For example:
```html
<input type="hidden" name="{{ api_settings.FORM_METHOD_OVERRIDE }}" value="OPTIONS" />
<button class="btn btn-primary js-tooltip" title="Make an OPTIONS request on the {{ name }} resource">OPTIONS</button>
```
Could be changed by:
```html
<button class="btn btn-primary js-tooltip" title="Make an OPTIONS request on the {{ name }} resource" name="{{ api_settings.FORM_METHOD_OVERRIDE }}" value="OPTIONS">OPTIONS</button>
```

P.S.
Sorry for this pause, I've been on vacation.

See also #591