Refactor to use multiple <form> tags

[matthijskooijman]

Currently, there is only one <form> tag in the entire page that is shared by all forms, which is not always ideal (i.e. in peer_reviewed_article, this requires adding a suffix to field names since all comment forms are submitted at the same time).

In #135, some changes were made to prepare for having multiple form tags (i.e. form submissions now explicitly reference their containing form and an explicit "default form" is tracked on HyphaHtmlDocument). This should be completed at some point.

A related improvement would be to turn submit buttons into normal submit buttons, rather than doing all form submits through the javascript hypha() function. This makes things simpler, and allows the browser to apply any validation (i.e. from required or type=email attributes on input fields, which are currently ignored, presumably because javascript forces form submission).