fix(tagging): allow tags to be added on space, comma, clickaway

[halfwhole]

Problem

Currently, adding tags requires the user to press enter, which can be non-obvious and unintuitive.

Solution

Allow tags to be added when the user presses enter, space, comma, or clicks away from the tags input.

But a slight ordering problem happens when the user clicks away from the tags input to the "create link" or "save" button at one go:
- When the user clicks "create link" on the create link form, first the tag input is added, then the link is created;
- When the user clicks "save" on the edit link form, first the link is saved, then the tag input is added.

edit: above problem is fixed, always adds tags before saving. but the solution might be a little unclean though

Some extra fixes: slight amendments to logic for disabling save/create link buttons

[thanhdatle]

LGTM