Add body to finding evidence responses

What does this MR do?

This MR adds the body column to Vulnerability Finding Evidence Responses, in accordance with issue #267489.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Database

Migration

== 20210512183309 AddBodyToFindingsEvidencesResponse: migrating ===============
-- add_column(:vulnerability_finding_evidence_responses, :body, :text)
   -> 0.0027s
== 20210512183309 AddBodyToFindingsEvidencesResponse: migrated (0.0027s) ======

== 20210512183310 AddLimitToFindingsEvidencesResponseBody: migrating ==========
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0003s
-- execute("ALTER TABLE vulnerability_finding_evidence_responses\nADD CONSTRAINT check_76bac0c32b\nCHECK ( char_length(body) <= 2048 )\nNOT VALID;\n")
   -> 0.0013s
-- current_schema()
   -> 0.0003s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE vulnerability_finding_evidence_responses VALIDATE CONSTRAINT check_76bac0c32b;")
   -> 0.0010s
-- execute("RESET ALL")
   -> 0.0006s
== 20210512183310 AddLimitToFindingsEvidencesResponseBody: migrated (0.0151s) =

Rollback

== 20210512183310 AddLimitToFindingsEvidencesResponseBody: reverting ==========
-- execute("ALTER TABLE vulnerability_finding_evidence_responses\nDROP CONSTRAINT IF EXISTS check_76bac0c32b\n")
   -> 0.0015s
== 20210512183310 AddLimitToFindingsEvidencesResponseBody: reverted (0.0110s) =

== 20210512183309 AddBodyToFindingsEvidencesResponse: reverting ===============
-- remove_column(:vulnerability_finding_evidence_responses, :body)
   -> 0.0026s
== 20210512183309 AddBodyToFindingsEvidencesResponse: reverted (0.0027s) ======

Related to #267489