Add reuse compliance

[AndyScherzinger]

• SPDX header
• CI check
• Readme badge
• issue template config from org

[AndyScherzinger]

@st3iny you might want to change https://github.com/nextcloud/calendar-availability-vue/pull/333/files#diff-380d439957b1ea48f4624d231e040af8bb63c44d41bb7d5bba5d7d2d3ea9c809 from AGPL3+ to MIT, else the lib is basically AGPL not MIT. I suspect this to be a simple c&p issue or file template issue.

[st3iny]

@st3iny you might want to change #333 (files) from AGPL3+ to MIT, else the lib is basically AGPL not MIT. I suspect this to be a simple c&p issue or file template issue.

Good point. It is meant to be distributed under the library's license, MIT.