Truncate all messages to valid length before sending.

...org/briarproject/android/blogs/WriteBlogPostActivity.java

...rg/briarproject/android/contact/ConversationActivity.java

...ect/android/introduction/IntroductionMessageFragment.java

...rg/briarproject/android/sharing/ShareMessageFragment.java

briar-api/src/org/briarproject/api/blogs/BlogConstants.java

...pi/src/org/briarproject/api/blogs/BlogSharingManager.java

...i/src/org/briarproject/api/forum/ForumSharingManager.java

.../briarproject/api/introduction/IntroductionConstants.java

...pi/src/org/briarproject/api/sharing/SharingConstants.java

...-api/src/org/briarproject/api/sharing/SharingManager.java

...-core/src/org/briarproject/blogs/BlogPostFactoryImpl.java

briar-core/src/org/briarproject/blogs/BlogPostValidator.java

briar-core/src/org/briarproject/feed/FeedManagerImpl.java

.../src/org/briarproject/introduction/IntroducerManager.java

.../org/briarproject/introduction/IntroductionValidator.java

.../src/org/briarproject/sharing/BlogSharingManagerImpl.java

...re/src/org/briarproject/sharing/BlogSharingValidator.java

...src/org/briarproject/sharing/ForumSharingManagerImpl.java

...e/src/org/briarproject/sharing/ForumSharingValidator.java

...core/src/org/briarproject/sharing/SharingManagerImpl.java