Replace itext 5.5 by itext 2.1.7 (prevent itext 5.5 AGPL license to supersede projects BSD-2 license)

[stefanhahmann]

User Story

Mastodon is licensed under BSD 2 license (https://github.com/mastodon-sc/mastodon/blob/master/LICENSE.txt). However, it currently references a library in the pom.xml (https://github.com/mastodon-sc/mastodon/blob/a965612e7890ec624232fc69c7431badd65c18d3/pom.xml#L113), that is licensed and AGPL, namely itext 5.5 (cf. https://mvnrepository.com/artifact/com.itextpdf/itextpdf/5.5.13.3).

The result of this is that the AGPL license of this library supersedes the original BSD-2 license of Mastodon.

Acceptance criteria

Either:

1. Change the license of the Mastodon project from BSD 2 to AGPL so that everything is sync.

Or:

1. Replace itext 5.5 by itext 2.1.7, which was the last itext release to not be licensed under (A)GPL (preferred)

Tasks

• [~] Replace itext 5.5 by itext 2.1.7
• [~] Replace PDF Export methods that needed itext 5.5 functionality by functionality that is also available in itext 2.1.7
• [x] Remove currently unused code that uses itext 5.5

[stefanhahmann]

Resolved by https://github.com/mastodon-sc/mastodon/commit/896373b79c346a6351e1d599347b05fb87e495af