[pgAdmin4][RM#4199] ENTER in Data Filter runs query

Attached is the patch to fix RM 4199 "ENTER in Data Filter runs query". To solve this problem I have remove the key mapping of the "OK" button, which is mapped to ENTER key and provided the alternative key combination that is "SHIFT + ENTER".

I have added Placeholder message for that in code mirror. Refer Data_Filter.png. 

Please review it.

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