bugfix: clear dialogs data upon reopening

[maximeBAY]

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Fixed dialogs that were not clearing previous data properly upon loading: Site, Comments, SiteAnnoucement, SiteContact

Also investigated rest of dialogs and the others were behaving the proper way

Closes #316

[Samuel-Therrien-Beslogic]

More of a "hide on opening" than a "clear on close". But whatever, as long as it works, this isn't the most robust part of the site.
As long as we keep improving the form validations, in #239, #240 and #242, that'll be the most important part.

[Samuel-Therrien-Beslogic]

Let's wait for #335 to be merged first to avoid bad merge conflicts on that PR. I'd rather have them here if any.