When posting comments, validate them as self-text and not regular markdown objects

When a user submits a comment, it is [validated as `VMarkdown`](https://github.com/reddit/reddit/blob/3bfe0a2513495f8e667f60ca3efce03f97990e51/r2/r2/controllers/api.py#L1234), which allows [`max_length = 10000` characters](https://github.com/reddit/reddit/blob/3bfe0a2513495f8e667f60ca3efce03f97990e51/r2/r2/lib/validator/validator.py#L574).  However, when a user *edits* a self-post or comment, it is [validated as `VSelfText`](https://github.com/reddit/reddit/blob/3bfe0a2513495f8e667f60ca3efce03f97990e51/r2/r2/controllers/api.py#L1184), which allows [`get_max_length: return self.max_length * 1.5` or 15000 characters]().


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It looks like the `POST_comment` validator didn't get updated properly a while ago, but the discrepancy went unnoticed until self-text max_length was extended.

* 2010-05-17: [`POST_comment`](https://github.com/reddit/reddit/commit/a402d48d#L6L604) and [POST_editusertext](https://github.com/reddit/reddit/commit/a402d48d#L6L570) now validate as VMarkdown.
* 2010-10-18:  [`POST_editusertext`](https://github.com/reddit/reddit/commit/37e2ba98/#L5L726) now validates as `VSelfText`, but [`POST_comment`](https://github.com/reddit/reddit/blob/37e2ba9892d1742f78ab496d8fc1d17797731078/r2/r2/controllers/api.py#L773) still validates as `VMarkdown`
* 2012-09-05: [`VSelfText` now has a max length of 1.5x characters of normal markdown text](https://github.com/reddit/reddit/commit/a3274399)