Bump bootstrap-vue from 2.0.0-rc.11 to 2.0.0-rc.12

Bumps [bootstrap-vue](https://github.com/bootstrap-vue/bootstrap-vue) from 2.0.0-rc.11 to 2.0.0-rc.12.
- [Release notes](https://github.com/bootstrap-vue/bootstrap-vue/releases)
- [Changelog](https://github.com/bootstrap-vue/bootstrap-vue/blob/dev/CHANGELOG-OLD.md)
- [Commits](bootstrap-vue/bootstrap-vue@v2.0.0-rc.11...v2.0.0-rc.12)

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