Create EXPORT_BLOCK_SIZE as constant

Used by csv and excel export procedures, to limit the
size of each working block of bugs.

Initial value is proposed after the potential errors for uncontolled
export:
A- Exceeding memory limit. Memory is used mainly for cached data.
Reported errors start to hapen at >10k bugs
B- Exceeding DB query limits: Mysql/others, limit is package size.
Oracle has a more problematic limit for 1000 elements in "IN" clauses".

A value of 500 is chosen as half of (B).