LPS-40606 OpenSocial Gadget Publisher updated to use <aui:button>

* The original ticket was that the buttons on this page were missing
  bootstrap styling. One previously proposed fix was to add the bootstrap
  class to the code for the buttons (see:
  https://github.com/noellekimiko/liferay-plugins/pull/6)
* However, the buttons really shouldn't use the `input` tag, since they
  are meant to be buttons, and they shouldn't have to have the class added
  manually because this makes maintenance harder.
* Instead, this fix uses the `aui:button` taglib since it will
  automatically format these buttons to have bootstrap styling. Down the
  road, these buttons will also require less maintenance if the bootstrap
  styling guidelines change -- the taglib will be updated and the changes
  will show up for these buttons, instead of having to update the css
  classes of these buttons individually.
* The `onClick` values (`portlet:renderURL` and `portlet:actionURL`) were
  moved out of the inline calls, since `aui:button` already expects
  javascript in its `onClick` value. This increases readability also.
* Also for `portlet:renderURL` and `portlet:actionURL`, a `var` attribute
  was added so that they could be referenced later in the `onClick`
  attributes for the `aui:button` elements.
* The `liferay-ui:message` tags were removed from the `value` attributes.
  In the `aui:button` taglib, the `value` attribute is already looked up
  in Language(-ext).properties file, replacing text as applicable. So this
  will format the text and translate it without having to use the
 `liferay-ui:message` taglib.
LPS-40606 - Source formatting
LPS-40606 SF