Support Dropbox API v2

[bprotas]

On September 28, Dropbox turned off the v1 API which this plugin was using; attempts to create issues with attachments or view existing attachments started failing in instances of Redmine that used this plugin to store attachments in Dropbox.

I've implemented V2 API support in this API, being as minimally invasive as possible. Generally, uploading, downloading, and deleting all had fairly obvious equivalents in the V2 API gem that I chose, and those changes were pretty straightforward.

It does get a little tricky around authorization, because the V2 API auth flow is just different enough than V1 to be confusing. App keys and secrets are now required in the plugin, and the plugin must be authorized via the web UI after they've been set.

Please take a look at the below; I don't have a ton of Redmine plugin experience, and I'm open to any feedback that you might have on how to improve.

Thanks!

[juxta73]

Hello! If the attachment is not in the dropbox, an error occurs when the task is will closed. And project can not be closed. Probably the system looks for an attachment in the dropbox and does not find it there and gets out an error

[bprotas]

Hi @juxta73 - It's unclear if you're saying this is an issue with my branch and that's why it's on this PR or an issue with the plugin in general; it sounds like the latter. Either way, do you mind creating a new issue or submitting a PR? It appears this repo might be abandoned - feel free to submit to my fork if you'd rather. Unfortunately, I don't have the bandwidth at the moment to investigate and fix the issue you mention as it's just not a case that I run into.