Add share button on proposals

[luanfonceca]

We have to scenarios for this issue:

1. After i submit my proposal is should be nice to popup a modal trying to make the author to share their proposal for the people to vote on it.
2. In the proposal action block (where we have the Update button) there is nice to have a button "Share" to open the same popup.

It is nice to use the same widget that we use today, just for be simple.

[anapaulagomes]

On it.

[anapaulagomes]

The number 2 was accomplished on #357. Feel free to tackle the number 1 (front end is not my thing, sorry 😐).