add `History::SetPreviousRevision`
add `History::Exists()` and `History::Get()`
TEST: adapt history unit test cases
revise `cvmfs_swissknife tag_create`
Refactor: `CommandCreateTag` for code reuse
add `cvmfs_swissknife tag_remove`
add assertion for upload
add `History::Tag::GetChannelName`
add implementation for `cvmfs_swissknife tag_list`
add `cvmfs_swissknife tag_info`
read only access based on signed manifest
merge info from new and old manifest
FIX: typo
make tag description optional
rewire `cvmfs_server list-tags`
add `check_tag_existence()` in cvmfs_server
remove tag handling from `cvmfs_server publish`
remove history handling from `cvmfs_swissknife sign`
add scaffold for `cvmfs_server tag`
add `cvmfs_server tag` to usage string