Merge pull request #77 from lyhypacm/persistence-cleanup

For #76 - delete maven configuration file and use gradle instead.
add java coverage plugin to help developers testing.
Merge pull request #79 from lyhypacm/testing-better

add java coverage plugin to help developers testing.
add idea plugin.
Merge pull request #80 from lyhypacm/testing-better

add idea plugin.
[FIX] Fix gradle jettyRun stoped immediately after run it
Merge pull request #81 from mzry1992/fix-gradle-jetty-run-issue

[FIX] Fix gradle jettyRun stoped immediately after run it
[UPDATE] Create team, contestTeam, teamUser table, and create patch script
[UPDATE] Remove ContestTeamInfo
[UPDATE] Add entities
[UPDATE] Set default value in database alert statement
[UPDATE] Modified register modal
[UPDATE] Update user editor
[UPDATE] Fix unique key in sql
[UPDATE] Add team service and team controller
[UPDATE] Add team register system
[UPDATE] Add custom hql query function
[UPDATE] Add user team list
[UPDATE] Send invitation where create new team
[UPDATE] Add allow state in team user