[Backport to 3.2.x][Fixes #7194] Append data to an existing layer (#7195)

[afabiani]

• [Fixes Append data to an existing layer #7194] Add new append_layer html

• [Fixes Append data to an existing layer #7194] Test helper utility for create single layer

• [Fixes Append data to an existing layer #7194] Add append data to existing edit layer html

• [Fixes Append data to an existing layer #7194] Flake8 formatting

• [Fixes Append data to an existing layer #7194] New layer_append url map

• [Fixes Append data to an existing layer #7194]Validation helper function for replace and appen layer

• [Fixes Append data to an existing layer #7194] Add view for layer_append feature

• [Fixes Append data to an existing layer #7194] Test coverage for new validation function

(cherry picked from commit b404e0d)

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