Notify users that their email will be used for bulk contact

[timlinux]

We should update the plugin upload environment with a little notice to indicate that uploading a plugin to our repo indicates explicit agreement to be notified via bulk email on important notices from QGIS for plugin authors.

I propose the following text:

Please note that by uploading a plugin to the official QGIS plugin repository, you agree that we will use your email to contact you. We will only contact you for matters relating to the management of plugins and will not make your email available to third parties for marketing purposes.

In addition we should add a proviso when a user uploads a plugin that the a condition of having the plugin hosted on http://plugins.qgis.org is that the author should be contactable and responsive to email correspondence from the QGIS plugin management team. I propose the following text:

By uploading your plugin to the QGIS plugin repository, you agree to keep your email address current and to be responsive to any correspondence we may send you regarding the management of your plugin. We require this in order to be able to provide our users assurance that the plugins we host are well maintained and will be fixed should serious issues arise during their use. We reserve the right to hide or remove plugins in cases where the plugin author is not contactable and there are issues reported about a plugin.

These messages should be displayed on upload of a plugin and should also be clearly visible on the user's landing page when they log in to the plugins platform.