[SPARK-8399] [STREAMING] [WEB UI] Overlap between histograms and axis' name in Spark Streaming UI

Moved where the X axis' name (#batches) is written in histograms in the spark streaming web ui so the histograms and the axis' name do not overlap.

Author: BenFradet <benjamin.fradet@gmail.com>

Closes #6845 from BenFradet/SPARK-8399 and squashes the following commits:

b63695f [BenFradet] adjusted inner histograms
eb610ee [BenFradet] readjusted #batches on the x axis
dd46f98 [BenFradet] aligned all unit labels and ticks
0564b62 [BenFradet] readjusted #batches placement
edd0936 [BenFradet] moved where the X axis' name (#batches) is written in histograms in the spark streaming web ui