Create a first version of the application #1

[oanalavinia]

• add maven structure

• add apis for: get file info, get file content, update file; these are used by Collabora server in order to access the edited files https://sdk.collaboraonline.com/docs/How_to_integrate.html#connection-to-the-file-storage

• have a button near attachments to edit them with Collabora

• have button at the end of attachments list to create a new file using Collabora; on click a modal is opened, to select the type of file (Document, Presentation or Spreadsheet) and the file name; an notification is displayed in case the file exists already, otherwise the user is redirected to edit the new file using Collabora

• add admin configuration page to specify the server address and to enable / disable the Collabora feature
• create a discovery endpoint, to get the urlsrc; this is needed by the Collabora editorin order to know which part of the editor to load https://sdk.collaboraonline.com/docs/advanced_integration.html?highlight=discovery#modifying-discovery-xml
• use the discovery endpoint to also create an authentication token for each file; a token is unique per file and attachment, has a valability of 1h and is destroyed when no window is using it anymore; a token is needed in order to authenticate the request from Collabora server https://sdk.collaboraonline.com/docs/How_to_integrate.html#authentication