D-11

v-2.0b
---
1. $a_categorized => change key to Category.id

    ==> done
D-11

v-2.0c
---
1. Article#index => add category id to param

    ==> done
D-11

v-2.0d
---
1. save history (except "content")

    ==> done
D-11

v-2.1a
---
1. fix: History#index,view => remove tags

    ==> done
D-11

v-2.1b
---
1. save content

    ==> done