1000 contributions PR

[harshkhandeparkar]

Fixes #[Add issue number here.]

Image Sequencer completed 1000 contributions 🎉. Changed the PR template to celebrate.

Newcomers don't fully understand where to put the issue number and end up putting the number in the square brackets like #[254]. This will no longer happen (hopefully)

Make sure these boxes are checked before your pull request (PR) is ready to be reviewed and merged. Thanks!

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Thanks!

[harshkhandeparkar]

@publiclab/is-reviewers