Proposal shared with confirmation process

[cdparra]

• [backend] Add a flag to the relationship between Contribution and User/NonMemberAuthor to store the confirmation by the user. By default set to FALSE to all shared with, and to TRUE for creators.
• [backend] In the email notifying the user about the proposal that was shared, include a token so that when the user opens the URL of the proposal, AppCivist can ask for confirmation.
• [backend] Add a share with confirmation service endpoint
• [frontend] Add a modal in the proposal page to ask users confirmation that they want to be part of it if they were shared with
• [frontend] Connect the confirmation share with modal to the backend, use the token to communicate the response to the backend
• [frontend] In the proposal page hide users/non-member-authors from the list of shared with people if their confirmation flag is false
• [maitenance] Send an email to all users who have been shared with a proposal asking them to log in, visit each of the proposals that have been shared with them, and confirm their participation.