[SPARK-3844][UI] Truncate appName in WebUI if it is too long

Truncate appName in WebUI if it is too long.

Author: Xiangrui Meng <meng@databricks.com>

Closes #2707 from mengxr/truncate-app-name and squashes the following commits:

87834ce [Xiangrui Meng] move scala import below java
c7111dc [Xiangrui Meng] truncate appName in WebUI if it is too long

(cherry picked from commit 86b392942daf61fed2ff7490178b128107a0e856)
Signed-off-by: Andrew Or <andrewor14@gmail.com>