Issue title: New GitHub Sponsors program + Bountysource integration not working
 Issue body: Hello everyone!
GitHub just launched the beta of its new Sponsors program. Basically, in the future, you will be able to send money to anyone on GitHub **free of charge**, as long as its associated to a repository, issue or PR. Also, for the next 12 months, GitHub will equal every donation! Details are here: https://github.blog/2019-05-23-announcing-github-sponsors-a-new-way-to-contribute-to-open-source/.
Concretely, what it means for OT is that a sponsor button could be added at the top of the repository, and up to four contributors could be listed for individual donations. The setup is automatic: https://help.github.com/en/articles/displaying-a-sponsor-button-in-your-repository.
@shun-iwasawa Could you look into this, please? There is also the Bountysource integration that is not working anymore, which is problematic because there are still active bounties. Thank you!
 Comments: 
 Comment 0: tagged this with 'resource' and 'roadmap' as funding of development is an ongoing concern.
 Comment 1: The Github Sponsors program is said to be live now but no developer to date has signed up to be sponsored.

Will keep this open for now and reference it via the primary Bounties Listing.  Then even when closed developers can reference it.


 Comment 2: @jpturcotte Did we ever discover why BountySource integration wasn't working?
 Comment 3: It seems to me that bountysource integration was/is working but only in specific cases where the reporter leaves the code for the bountysource plugin (added via template) in the report.  If they remove the entry then the plugin isn't called.

Will investigate.
 Comment 4: I have to ask...

I don't suppose you could add a 'Join' option where if the snapping of control points is to the end of a spline/line then the ends would automatically be joined.  @shun-iwasawa just now added some code relating a little to that with the Tape Tool.  That PR will not be merged until after v1.4 goes final release also.
 Comment 5: wrong thread
 Comment 6: The following is the markdown that appears necessary for a post to be linked to BountySource:

```
<bountysource-plugin>

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/39331590-suspicious-incorrect-use-of-abs?utm_campaign=plugin&utm_content=tracker%2F33713530&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F33713530&utm_medium=issues&utm_source=github).
</bountysource-plugin>
```
 Comment 7: Here's what it looks like:

<bountysource-plugin>

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/39331590-suspicious-incorrect-use-of-abs?utm_campaign=plugin&utm_content=tracker%2F33713530&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F33713530&utm_medium=issues&utm_source=github).
</bountysource-plugin>
{'source': 'GithubIssues', 'repo_name_with_owner': 'opentoonz/opentoonz', 'issue_id': 448477286, 'repo_stars': 3868.0}