basic working version of adminSessions view
improve function:
1.add floating back to top button
2.add hover menu
3.change show/hide in admin sessions page to sliding style
4.import jqueryui
1. add handling of empty ongoing sessions situation
2. make other sort buttons reset when one sort button is clicked
3. make hover menu insensitive to hovering when it is being animated
first working version:

using range to search for sessions

How it works:

1. first time page load: range start and end will be set to the current time with time zone UTC,
    all sessions going on at this time will be shown
2. when given a range:
    2.1 convert the given range with given given zone to a specific time zone (starting from UTC - 12 to UTC + 13)
    2.2 go to the datastore find all the sessions that overlap with this converted range
    2.3 loop 2.1 and 2.2 until all the time zones are checked
1.update querying method
2.add presentation of searching range on top of adminSessionsPage
temp push
add status msg for invalid filter range
Merge remote-tracking branch 'origin/master' into 73359266
fix merged test
dev green
refactor code
change sort icon
1. apply SLAP
2. fix typo
3. move convert-to-default-date-format method into TimeHelper
wait for the page to finish the animation
Merge branch 'dev' into AccessControl
Merge branch 'dev' into AccessControl

Conflicts:
	src/test/java/teammates/test/pageobjects/InstructorCourseEditPage.java
Merge branch 'dev' into AccessControl
Merge branch 'master' into AccessControl
Merge branch 'dev' into AccessControl
1.add time filter for log
2. add timeHelper test
3. add related reference
remove removedata in commentsUITest