Widths set on Blocks Everywhere in Forums breaks layout

Unfortunately the styles added in https://github.com/Automattic/blocks-everywhere/pull/201 to correct the editor width break the layout in a couple of scenarios:

1. We launched a new Forums theme which has a wider content layout, so the 620px width isn't compatible
2. On mobile devices the 620px width causes horizontal overflow (in both the old and new themes)

### Screenshots

| New theme desktop | New theme mobile | Old theme mobile |
|-|-|-|
|  |  |  |

### Suggested fix

This seems to work in all the scenarios above:

```scss
.gutenberg-support #bbpress-forums fieldset.bbp-form .blocks-everywhere {
max-width: 100%;
min-width: 100%;
}
```

Locate the styles added by PR #201 and inspect the `.gutenberg-support #bbpress-forums fieldset.bbp-form .blocks-everywhere` rule. Apply the issue's suggested sizing change, then verify the new wide theme and both old and new mobile layouts no longer overflow.

Written by the indexing model from the issue text.