Add auto-linking URLs via react-showdown

[ddrscott]

• A short explanation of the proposed change: Support add auto-linking URLs via react-showdown. reach-showdown is a Markdown to HTML converter so it has many more features than auto-linking. All links will be created with target=_blank they open in a new tab/window and don't interfere with the retro board.

• An explanation of the use cases your change solves: Support links for more details to specific cards.

• Links to any other associated PRs

• [x] I have reviewed the contributing guide

• [x] I have made this pull request to the master branch

• [x] I have run all the tests using ./test.sh.

• [x] I have added the copyright headers to each new file added

• [x] I have given myself credit in the humans.txt file (assuming I want to) not needed.

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[ddrscott]

This PR covers part of this issue: https://github.com/pivotal/postfacto/issues/296