Slack bridge interprets message in < brackets > as link

[Bustel]

Describe the bug
If somebody writes a message in slack in <brackets> it comes out as a broken link in matrix.

To Reproduce
Steps to reproduce the behavior:

1. Write message in slack channel: <test message>

Expected behavior
Normal messages in brackets should be treated as text.

Screenshots
If applicable, add screenshots to help explain your problem.
View from Element:

View from Slack:

Desktop (please complete the following information):

• OS: Manjaro
• Browser: Firefox 81
• Version: Latest Docker container of synapse + slackbridge

[tadzik]

Fixed in #781